Rumoured galpal Amanda Beard on Phelps: No Thanks! What was different was the political ads that appeared — or didn’t — beside the story.

Readers who had visited Barack Obama’s website received three Obama ads alongside the gossip. “Help Elect Barack Obama President of the United States” and “Visit the Barack Obama website,” they said.

Readers who hadn’t visited his site didn’t see a single Obama pitch.

How did the campaign know which readers to send ads to? Although both the Obama and John McCain campaigns are reluctant to discuss details, the ability to identify sympathetic voters based on their internet habits, and then to target them with ads, is a defining aspects of the 2008 presidential campaign.

Digital advertising networks and large web companies like Yahoo and Microsoft are using web behaviour — which news articles people read, which blogs they visit or what search terms they enter — to target voters who may be sympathetic to a certain cause. Using a method known as “sentiment detection”, some companies even boast of telling whether the blog you go to is for or against the Iraq war.

“During a get-out-the-vote drive, you don’t want to get out the wrong vote,” said Diane Rinaldo, political advertising director at Yahoo. The advertising techniques, known as “behavioural targeting” and “retargeting”, have raised alarms from some privacy advocates, who say no one should unwittingly have their political leanings analysed as they use the web, or be tracked for the delivery of political ads.“The web has been hailed for creating new opportunities for political expression, but there is this dark underside to it,” said Jeffrey Chester with the Center for Digital Democracy. Advocates of the practice, say its use in the political world is comparable to traditional direct-mail campaign practices. By contrast, most online targeting is directed to a web browser, and the name and home address of the target is unnecessary.

Guessing how a person might vote — and whether they might be receptive to a pitch — has long been part of the science of political marketing. Both presidential campaigns are using “retargeting” to send ads to people who visited their websites but who didn’t leave their name or e-mail address.To track those visitors even after they’ve left, the site places a small file, known as a cookie.When that person visits another site, an advertising system can send a tailored ad after detecting the cookie.That’s how the Obama campaign can send an ad to a person long after they’ve visited the Obama site, even when their mind is on something far afield from politics — like Phelps and Beard.The cookie might even indicate a user’s interests, allowing the campaign to further tailor an ad. “If you responded to a certain kind of ad, we could hit you with a similarly themed ad at another time,” said Michael Palmer, the eCampaign director for McCain.

Gathering data on all the web visits people make, the company can then present a political campaign with “buckets” of voters described as Republican or Democrat, conservative or liberal, and by what specific issues — the person — identified by a cookie on their browser, may be interested in. Similarly, Yahoo collects information about the 140 million unique monthly visitors to its sites.